1) Field of the Invention
The present invention relates to a surface tension measuring device and method for measuring an interface property in an interface produced between air and a liquid or between different liquids, and more particularly for measuring surface tension.
2) Description of the Related Art
Surface tension is generated at the interface produced between air and a liquid or between different liquids. Surface tension is a fundamental physical property. The behavior of a liquid can be predicted by measuring the surface tension, which is an important physical parameter for industries involved with mixing, circulating, or otherwise handling liquids. Interface properties (wettability, interface contamination, and the like) related to surface tension are also important parameters.
A droplet method such as that disclosed in Patent Document 1 is a conventional method for measuring surface tension. This method involves observing droplets dropped from a narrow tube, but is not regarded to be simple because an image captured with a video camera is processed, and the pressure is controlled when a test material is drawn into the narrow tube in order to control the dropping of the droplets. Additionally, the sample to be measured must be introduced into the narrow tube and the amount of sample liquid inevitably increases. Furthermore, the narrow tube and other parts of the fluid supply system must be cleaned and there are many other disadvantages. Another conceivable method is one that uses an optical fiber as disclosed in Patent Document 2. This method is characterized in that a liquid sample material is deposited on the distal end of an optical fiber disposed in the optical path of a signal arm of a Michelson interferometer or another type of spectrointerferometer, and the physical properties of the liquid test material are measured from the interference signal obtained when there is interference between a reference light and a signal light reflected from the surface of the liquid test material. Similarly, this method cannot be regarded as simple because it requires a spectrointerferometer. Additionally, there is a drawback in this method in that a liquid with high surface tension such as water is difficult to uniformly deposit on an optical fiber.
Patent Documents 3 and 4 are prior art of the present inventors. These documents describe an air bubble detection probe for measuring the parallel speed, interface speed, air bubble chord length, and air bubble volume fraction using a fiber with an inclined distal end surface, but there is no description of measuring the surface tension at an interface. The interface deformation when the interface has made contact with the distal end of the fiber is regarded as negligible (paragraph 0011 of Patent Document 3) and consideration has not been given to the effect of surface tension.    Patent Document 1: Japanese Laid-open Patent Application No. H 8-152396    Patent Document 2: Japanese Laid-open Patent Application No. 2005-249696    Patent Document 3: Japanese Laid-open Patent Application No. 2000-136962    Patent Document 4: Japanese Laid-open Patent Application No. 2006-226703